(1) Field of the Invention
The present invention generally relates to a method and apparatus for an image sensor. In particular, the present invention relates to controlling a data flow from an image sensor to an interface.
(2) Background of the Invention
Complementary metal oxide semiconductor (herein CMOS) sensors have been of interest as replacements for charge-coupled devices (herein CCD) in imaging applications. CMOS sensors promise lower power and simpler system level design through fewer power supply voltages and higher functional integration. These factors contribute to lowering system cost while providing for a potential “camera on a chip.” Such features are highly desirable. For example, in camcorders or digital cameras, the devices may be reduced to a size of a TV remote control while allowing higher resolution color images to be recorded for hours.
Typically, in a CMOS sensor device, a CMOS sensor would generate a high frame rate in comparison with a processing rate of a data processing circuitry which may be an internal circuitry of the CMOS sensor device or it may be external such as a computer. As an example, the CMOS sensor may be generating an output of about 30 frames per second. However, the data processing circuitry may be processing at a rate of only 10 frames per second. In this instance, a data overflow will result at the point of the data processing circuitry. One prior method involved designing the CMOS sensor such that the data flow rate from the CMOS sensor matched the processing rate of the processing circuitry. However, the disadvantage is that the CMOS sensor matched to one data processing circuitry may not be used with other data processing circuitries, which may have different data processing rates. In addition, a data processing circuitry may have to process data at several different processing rates dependent on system performance parameters which cannot be covered by a CMOS sensor with one data flow rate.
Another prior method involved buffering the data from the CMOS sensor into a temporary data storage such as random access memory (herein RAM) in contemplation of various data processing rates of the data processing circuitries. From the RAM, a data processing circuitry accessed data at its data processing rate. However, this method assumes that the rate of data output from the RAM will be high enough such that the rate of data input to the RAM will not overload the RAM. Thus, the method is limited to a small range of data output processing rate. Further, in this method, large banks of RAMs are required. However, RAMs are expensive and increases the overall cost of a system using CMOS sensor, making it undesirable in a highly competitive environment such as the camera market. Yet, another method involved discarding the excess data flow from the CMOS sensor when the data processing circuitry became overloaded with datum. However, in portable devices such as a digital camera or digital camcorder, precious battery power is wasted generating unnecessary datum shortening the operating time of the devices.
Accordingly, what is needed is a method and apparatus for controlling the data transfer rate of a CMOS sensor such that the sensor may be used with a variety of data processing circuitries with various data processing rates and further does not unnecessarily waste electrical power.